A structure operating in a seaway is subjected to oscillatory wave forces. In response to these waves forces, a floating structure can compliantly move in up to six principal degrees of freedom. Three of the degrees of freedom are translational, namely heave (vertical), surge and sway (horizontal) motions of the structure. The other three degrees of freedom are roll, pitch and yaw motions which correspond to rotational motions about the structure's principal axes. A column-stabilized, semisubmersible structure is generally free to compliantly move in all its six degrees of freedom. However, in some semisubmersible designs, noncompliant restraint might be imposed to one or more of the degrees of freedom such as in the case of a Tension Leg Platform (TLP) which is rigidly restrained in heave by its vertical tethers.
It is a well known practice to anchor semisubmersible floating structures over a seabed site using a spread type mooring system. The structure is usually kept on location for the purposes of conducting various types of operations including hydrocarbon drilling and production operations.
The spread type mooring system, often called "catenary" mooring system, is adapted primarily to restrain the horizontal motions of the structure and keep it over the desired seabed site, within allowable limits, by resisting the prevailing environmental forces. The design of a spread type mooring system is generally well known in the art and is comprised of a plurality of mooring means arranged in a radial pattern around the perimeter of the structure. Each individual mooring means generally comprises an anchor and a mooring line, said mooring line typically comprising a wire cable or a combination of a wire cable and anchor chain.
When environmental forces, such as wind, current and waves, act against the structure to move it away from its original location, spread mooring systems develop a net horizontal force called the "restoring" force which restrains unwanted movement of the structure and ultimately "restores" the structure to its original location. The restoration force is developed by the increasing tensions in the mooring lines located on the side of the structure experiencing the environmental forces (the "windward" side), as those lines become increasingly taut due to the movement of the structure, coupled with decreasing tension in the mooring lines located on the leeside of the structure.
It is also known that the station-keeping properties of the mooring system can be improved by the addition of weights ("clump weights") attached to the mooring lines to obtain a taut catenary mooring system. U.S. Pat. No. 3,903,705 to Beck discloses the use of clump weights attached to mooring lines, said clump weights being intended to remain at least partially resting on the seabed under normal environmental conditions.
A floating structure, in conjunction with its mooring system, effectively behaves like a spring/mass system and as such is subject to excitation of its principal degrees of freedom. Excitation of any of the structure's six degrees of freedom is imparted on the structure mostly within two principal frequency bands, a first band corresponding to the range of wave frequencies having the dominant wave energy and a second band centered about the resonant frequency of each of the degrees of motion.
Structure response within the range of frequencies containing the dominant wave energy is controlled by well-known hydrodynamic principles. A particular concern associated with the design and operation of compliant, floating structures is the possibility of resonant excitation of one or more of the structure's degrees of freedom. Potentially large resonant motions can occur whenever the seaway contains wave energy near one or more of the natural periods of motion of the structure. The resulting amplitude of motion due to resonant excitation is very dependent on the amount of damping in the system, as provided by either passive or active sources. It is very desirable to provide the structure with means to avoid and/or reduce resonant excitation.
All real spring mass systems possess a finite amount of natural damping. In the case of a floating structure, the principal source of natural damping is provided by viscous hydrodynamic effects on the vessel. More recently, it has been recognized that the viscous forces developed by the changing catenary geometry of the mooring lines can also be an important passive source of damping structure motion. Since the spread mooring system is deployed at an angle in the vertical plane, the damping forces generated by the mooring system have vector components of force acting in the vertical and horizontal directions and can therefore dampen all six degrees of freedom.
U.S. Pat. No. 4,167,147 to Bergman describes a floating structure having a variety of arrangements for actively and passively producing velocity damping, i.e., antiheave forces that are proportional to the heave velocity of the structure. All of the methods described by Bergman supplement the natural damping of the structure by the addition of mechanical systems on board the structure arranged so as to generate a damping force only in the vertical direction
Pending U.S. patent application by Petty, et al, Ser. No. 07/355,431, now U.S. Pat. No. 4,936,710, describes a mechanical system adapted on the mooring system to generate (coulomb) damping forces. Since the damping forces in accordance with Petty's invention are generated in conjunction with the mooring system, all six degrees of freedom, including heave and surge motions will be reduced.
Common to all the methods described above to supplement the natural damping characteristics of the structure and reduce structure motions is the need for modifying existing mechanical means or the addition of mechanical systems such as hydraulic cylinders, thrusters and so forth, all of which require various levels of on-going maintenance to ensure that the systems will function properly when required, usually in the event of an extreme storm. Further, various levels of operator intervention may be required, depending on the system, to operate the system through the passage of the storm. The need for maintenance of mechanical systems and/or special operating procedures introduces reliability risks, especially when operators are required to make critical decisions or maneuvers in very adverse weather conditions.